Voters, take heart. You may be able to turn on your televisions and open your mailboxes this spring without being inundated with slashing political propaganda, at least from Democratic groups.

On Wednesday, Representatives Edward J. Markey and Stephen F. Lynch signed a pact designed to keep outside groups from running ads or sending mailers in their race for the Democratic nomination for US Senate. The so-called People’s Pledge aims to hold the candidates accountable for the messages voters receive and to limit the shadowy attacks that characterize modern political campaigns.

The pact is modeled on a similar agreement that Elizabeth Warren and Scott Brown pioneered in their Senate race last year. To the surprise of many, that pact held up and successfully kept outside groups from entering one of the nation’s most competitive and closed watched Senate races.

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The agreement signed by Lynch and Markey, like the one hammered out by Brown and Warren, does not legally prohibit outside groups from running ads. It says, however, that if a group runs an ad on behalf of one of the candidates, that candidate has to donate 50 percent of the cost of the ad campaign to a charity of his rival’s choosing.

The penalty was the same in the Brown-Warren race and was twice triggered early in the contest, prompting Brown to donate a total of $36,000 to an autism charity.

The Lynch-Markey pact does not cover automated phone calls, meaning outside groups can still interrupt dinner with recorded messages blasting one candidate or another. It does, however, cover ads on TV, radio, and online, as well as propaganda sent through the mail, which was not covered under the Brown-Warren agreement.

Lynch and Markey called on the Republican candidates to sign the same pledge. The GOP primary pits newcomer Gabriel Gomez, a former Navy SEAL, against state Representative Daniel Winslow, a former aide to Governor Mitt Romney.

“Outside interest groups have no place in Massachusetts elections,” Lynch, a South Boston Democrat, said in a statement. “This race should be decided in debates and on the stump, not by third party advertisements or special interest mailers. I urge the Republican, Libertarian, unenrolled, and any other candidates to join us in this pledge.”

Markey, a Malden Democrat, echoed the sentiment in his own statement .

“Outside money has no place in the Massachusetts Senate race,” he said. “This election should be focused on issues, not outside-group attack ads. I urge all candidates in this race to join us in committing to the people’s pledge and say no to the outside special interests who want to influence this election.”